1. Field of the Invention
This invention relates to a substantially quadrilateral-shaped seat back frame for a vehicle seat, which is assembled from components which are each made of a thin plate of ultra-high tensile strength steel and formed into a predetermined shape by press-machining the thin plate.
2. Description of the Prior Art
Generally, a seat back frame for a vehicle seat comprises a main pipe of a substantially inverted U-shape in outline, an upper plate member bridged between upper regions of both side portions of the inverted U-shaped pipe and mounted to the upper regions of the both side portions of the inverted U-shaped pipe by arc welding, side plate members mounted to the both side portions of the inverted U-shaped pipe by arc welding, and a lower pipe bridged between lower regions of the both side portions of the inverted U-shaped pipe and mounted to the lower regions of the both side portions of the inverted U-shaped pipe by arc welding. Each of the upper plate member and side plate members is formed into a predetermined shape by cutting a steel plate. The seat back frame is assembled from the components into a substantially quadrilateral-shape.
The conventional seat back frame is heavy in weight and weak in mechanical strength. As discussed above, the upper plate member, the side plate members, and the lower pipe member are mounted to the inverted U-shaped main pipe by arc welding. To carry out the welding while assuring a dimensional precision is troublesome.